With the widespread use of inverter equipment, increasing amounts of harmonics are generated due to the loads of rectifiers. This increases the possibility of overheating or breakage of phase advance capacitors and transformers of power systems due to the harmonic current. Therefore, there is a demand to suppress harmonics of the rectified current obtained on the basis of a three-phase power supply (for example, JIS standard JISC61000-3-2).
In a common three-phase rectifier circuit, six steps appear in one period of the current waveform. In general, with an n-step current waveform, the lowest-order harmonic is of the (n−1)th. Then, the harmonic content with respect to the fundamental wave is 1/n, and so the fifth and seventh harmonics significantly appear.
Accordingly, techniques for suppressing harmonics are proposed, for example by Non-Patent Documents 1 and 2 and Patent Document 1. These schemes adopt a technique called multiple rectification, and Non-Patent Documents 1 and 2 disclose techniques in which each of polyphase currents generates 12 pulses in one period, and two rectifier circuits are connected in parallel to the load (this technique is hereinafter referred to as “12-pulse rectification”). Also, Patent Document 1 discloses a technique in which each of polyphase currents generates 18 pulses in one period, and three rectifier circuits are connected in parallel to the load (this technique is hereinafter referred to as “18-pulse rectification”). Adopting these techniques eliminates the fifth, seventh, or eleventh, thirteenth harmonic components.
Non-Patent Document 2 provides an additional coil to an interphase reactor connecting the outputs of two rectifier circuits together, in order to improve the current waveform in 12-pulse rectification. Alternating current is applied to the coil, so as to inject a current into the interphase reactor. This improves the current waveform inputted to the transformer for obtaining the 12 pulses of current.
However, injecting a current into an interphase reactor with an additional coil, as described in Non-Patent Document 2, requires applying a complicated waveform to the coil. Non-Patent Document 2 avoids such a complicated waveform, and applies a triangular wave to the coil, but then distortion remains in the input current as a natural result. Also, the device for injecting current into the interphase reactor is configured complicatedly, or a separate voltage source is required.
Choppers described later in the preferred embodiments are introduced in Non-Patent Document 3.
Patent Document 1: Japanese Patent Laid Open No. 59-15475 (1984)
Non-Patent Document 1: Hisao Matsumoto, “Twelve-pulse Converter with Autotransformer Connection”, The Institute of Electrical Engineers of Japan, Transactions B, vol. 96, no. 8, pp. 406-412
Non-Patent Document 2: Sewan Choi, P. N. Enjeti, H. Lee and I. L. Pitel: “A new active interphase reactor for 12-pulse rectifiers provides clean power utility interface”, IEEE Trans. Ind. Applicat., vol. 32, no. 6, pp. 1304-1311 (1996)
Non-Patent Document 3: Takao Hirasa, “Power Electronics”, Kyoritsu Shuppan Co., Ltd, pp. 77-90, 1992